Growth of non-polar and semipolar GaN buffer layers for device applications
Most of the excellent non- and semipolar device results were achieved on GaN bulk substrates (see Sections 8.5 and 8.6). Therefore we will review GaN growth on bulk substrates. However, for mass production, low-cost large-area substrates are desirable. These must be heteroepitaxial substrates, since low-cost large-area semipolar bulk GaN substrates are currently not available.
Growth of GaN on low-defect bulk GaN substrates
The growth of GaN layers on low defect density GaN bulk substrates is governed by surface diffusion processes. However, interaction with threading dislocations can lead to the formation of macroscopic defects (Wernicke et al., 2011), even at the low density of ~ 105-1 x 107 cm~2 present in bulk substrates (Fujito et al., 2008). These can be avoided using substrate miscut or proper growth conditions that limit the adatom diffusion length.
In Fig. 8.6 the macroscopic surface morphologies (Normarski contrast images) of on-axis (10l2), (10l1), (1122), (2021) and (1010) GaN layers grown with different temperatures and pressures are shown. The (1010) surfaces exhibit pyramidal structures for all growth parameters. These form at screw dislocations when a
Fig. 8.6. Normarski contrast micrographs of (1012), (1011), (1122), (2021), and (1010) GaN surfaces grown at 980°C and varied reactor pressure.
pinned surface step leads to a local enhancement of the growth rate (Farrell et al., 2010). On m-plane GaN this pyramidal surface structure can be avoided using miscut substrates (Hirai et al., 2007b). The miscut leads to a defined step orientation and a sufficiently high step density to completely hinder the formation of growth spirals. Also, for layers grown on semipolar GaN macroscopic features can be observed: triangular pyramids on (1011), shallow round pyramids on (1012), and elongated hillocks on (1122). For (1011) triangular pyramids, a dark spot was found in the center which indicates the presence of a dislocation (not shown). These are strong indications that dislocations cause macroscopic features also in semipolar GaN. However, by changing the growth parameters these features can be reduced or even eliminated. Two different classes of plane can be distinguished. The low-indexed (1011) and (10l2) smooth with increasing reactor pressure, whereas the higher-indexed (1122) and (2021) planes are smoothest for low reactor pressures. A similar behavior is known from the stability a-plane vs. m-plane facet (Sun et al., 2008). The m-plane facet is more stable, and smooth layers can be grown at various growth conditions. The a-plane is rather unstable, and for a full coalescence low reactor pressure and high temperatures are required (Chen et al., 2004).
Looking at the microscopic surface morphology of (1122) and (2021) layers, distinct stripe patterns are visible (Fig. 8.7). For the (1122) surfaces, undulations with short period are found along , and with a longer period along  (Ploch et al., 2012a). On (2021) surfaces, undulations with short period are found along [10l4], which is the projection of the c-plane onto the growth plane, also denoted as c' (Ploch et al., 2012c). No undulations were found along . These stripe patterns are the result of anisotropic surface diffusion and provide a less stable surface (in comparison to the (1011) and (10l0) surfaces). Surface diffusion activation energies were calculated for (1010) and (1120) GaN (Lymperakis
Fig. 8.7. AFM image of a) (1122), and b) (2021) surfaces. Images are courtesy of S. Ploch (Technical University, Berlin).
and Neugebauer, 2009), showing that adatoms can diffuse easily along closespaced nitrogen-atom rows. For (1010) these are oriented along , and the activation energy for diffusion is 4.5 times lower than along the perpendicular . For a-plane the closely spaced nitrogen-atom rows are aligned parallel , and the activation energy for diffusion along the perpendicular  direction is higher by a factor of two. Geometrical considerations suggest the same for the (101l)-type tilted m-plane surfaces (like (1011), (1012), and also (2021)) and (112l)-type tilted a-plane surfaces (like (1122)) (Dinh, 2012). In fact, anisotropic diffusion can explain the formation of undulations on both (1122) and (2021) surfaces. In this case the undulation is formed by adatom diffusion, and its period is proportional to the diffusion length. For (1122) surfaces the undulation with high spatial frequency is oriented along  which exhibits a high diffusion barrier on (112/)-type surfaces. Longer diffusion occurs along . In fact, from the temperature dependence of the undulation periods, activation energies of 0.8 eV and 1.3 eV along  and , respectiviely, were derived (Ploch et al., 2012a). For (2021) the undulations are oriented along the c' direction, which exhibits a high diffusion barrier on (101/)-type surfaces. However, in order to explain the occurrence of undulations at all, the facet stability has to be taken into account (Ploch et al., 2012c). As mentioned above, facets that exhibit a very high stability are the (1011) and (1010) facets. Therefore it is energetically favorable to form areas with (1011) and (1010)-like surfaces. For (1122) surfaces, the 1011 plane are titled by ± 26° towards . Therefore undulations along  are stabilized by the presence (1011) micro-facets. For the (2021) surface orientation the (1011) plane is titled by 13° towards , and the (1010) plane is tilted by 15° towards . This stabilizes undulations along .
For heteroepitaxial (1122) GaN the same dependencies were found. However, the presence of stacking faults deteriorates the surface (Ploch et al., 2012a), as was also reported for m-plane GaN (Hirai et al., 2007a).